1. Technical Field
The present disclosure generally relates to semiconductor structures, and particularly to a multi-quantum well structure and a light emitting diode (LED) structure having the multi-quantum structure, wherein the multiple-quantum well structure has at least one composition grading barrier layer.
2. Description of the Related Art
LEDs have many advantages, such as high luminosity, low operational voltage, low power consumption, compatibility with integrated circuits, faster switching, long term reliability, and environmental friendliness which have promoted their wide use as a light source.
A conventional LED structure includes a multi-quantum well structure. The multi-quantum well structure includes a plurality of barrier layers and a plurality of well layers alternating with each other, wherein the well layers are spaced by the barrier layers. An energy band gap between each conduction band and a corresponding forbidden band of the well layer is smaller than that of between each conduction band and a corresponding forbidden band of the barrier layers. Accordingly, the electric holes moving from a P-type semiconductor layer to an N-type semiconductor layer and electrons moving from the N-type semiconductor layer to the P-type semiconductor layer are confined at the well layer since the barrier layers having greater energy band gap, and photons are obtained in the well layer to emit when conjunctions of the electrons and the electric holes occur.
However, in the conventional multi-quantum structure, moving speed of the electric holes is much slower than that of the electrons since the electric holes are heavier than the electrons. Accordingly, the electrons and the electric holes are not evenly arranged in the well layer, and an emitting efficiency of the LED structure is low.
Therefore, it is desirable to provide a multi-quantum well structure which can overcome the above-described problems.